1. Field of the Invention
This invention relates to a method and apparatus for measuring surface carrier recombination velocity and surface Fermi level, and particularly to a method and apparatus for evaluating the carrier recombination velocity and the surface Fermi level at a semiconductor surface precisely at the same time in a non-destructive and contactless manner using modulation spectroscopy belonging to spectroscopic measurement technology.
2. Description of the Related Art
In general, the characteristics and reliability of semiconductor devices are closely related to surface and interface states of the epitaxial (herein below abbreviated as “epi”) layer structures. The term “semiconductor” means all kinds of solid materials the carrier concentration of which can be controlled by a doping technique. Further, when being referred to simply as “surface” hereinafter, it also includes the meaning of “interface.” For example, the surface/interface states affect a reduction in reliability, an increase in noise, frequency dispersion, and breakdown voltage in field effect transistors (FETs). They also increase a recombination current in heterojunction bipolar transistors.
In a surface-emitting laser diode (LD), a threshold current and differential quantum efficiency are related to the surface state. For these reasons, the quantitative evaluation of surface states is considered to be a key to the development and mass production of high-performance and high-reliability devices.
Photoreflectance (hereinbelow abbreviated as PR) spectroscopy is a kind of non-destructive, contactless spectroscopy based on modulation techniques and has recently been attracting attention as optical evaluation techniques for semiconductor surfaces (for example, see Japanese patent laid-open application No. 2002-340675). One of the reasons arises from the fact that PR spectroscopy is sensitive to the electric field strength. For example, in the spectrum of a semiconductor sample having a built-in electric field obtained by PR modulation spectroscopy, an oscillation pattern, called a Franz-Keldysh (hereinbelow abbreviated as FK) oscillation, appears near the critical point of the optical transition energy.
Electro-optic energy corresponding to a period of FK oscillations is determined by the surface electric field strength. In particular, since the surface electric field of the semiconductor is generated by a phenomenon called “surface Fermi level pinning”, the measurement of FK oscillations can be considered as one of the methods for estimating surface Fermi level. For example, as a practical matter, Shen et al. measured the PR spectrum of an epi structure consisting of an undoped AlxGa1-xAS (i-AlxGa1-xAS) layer, an n-type AlxGa1-xAs (n-AlxGa1-xAs) layer, and an n-type substrate to obtain the Fermi level of the i-AlxGa1-xAs layer. See Appl. Phys. Lett. 57, 2118 (1990).
The parameter, the so-called surface recombination velocity, is another important parameter that characterizes the semiconductor surface. This is a parameter that characterizes the recombination of carriers at the surface, and it affects the performance of the semiconductor device much as the surface Fermi level does.
Measurement methods other than the above-mentioned PR spectroscopy have been used to evaluate the surface recombination velocity. For example, there is a widespread method of performing time-resolved measurements on photoluminescence (hereinbelow abbreviated as PL) from the semiconductor and analyzing the decay profile of PL intensity plotted as a function of time after optical excitation to determine the surface recombination velocity.
In order to evaluate the surface Fermi level from the FK oscillations by a conventional measurement method, it has been considered necessary to eliminate a photovoltaic effect caused by continuous probe beam irradiation because the photovoltaic effect reduces the surface electric field strength. In order to suppress the photovoltaic effect, for example, in the above-mentioned measurement method proposed by Shen et al., the probe beam is defocused on the sample surface so that the density of the probe beam power is 0.1 μW/cm2 or less. Such a very weak probe beam, needless to say, causes a signal-to-noise (S/N) ratio reduction, resulting in difficulty in the PR measurement.
On the other hand, there are also problems in the method of evaluating the surface recombination velocity. One of the problems originates from the fact that in the analysis of the decay profile, it is difficult to separate the surface carrier recombination velocity from the carrier recombination in a semiconductor crystal. Such a method based on PL spectroscopy, of course, is inapplicable to measure the materials whose PL intensity is weak.
Since both of the above-mentioned two parameters, surface Fermi level and surface recombination velocity, reflect the surface state, it is considered that they correlate with each other. However, as mentioned above, the Fermi level and the surface recombination velocity have been conventionally measured by completely different methods. This means that the values of the Fermi level and the surface recombination velocity are obtained under different measurement conditions. Therefore, there is a fundamental problem that the discussion on a correlation between the two parameters acompanies with uncertainty resulting from the difference in the measurement. It is evident that two different kinds of measurements are needed to evaluate the two parameters. This results in an increase in evaluation time and cost, which prevents the precise surface design of semiconductor devices.
The present invention has been made to solve the above-mentioned problems. It is a purpose of the present invention to provide a modulation spectroscopic measurement and analysis method, which can determine not only the surface Fermi level but also the surface recombination velocity even from FK oscillations affected by a relatively high-power probe beam. It is another purpose of the present invention to provide an apparatus suitable for this method and a sample structure suitable to be evaluated.